1. Field of the Invention
The present invention relates to an inkjet apparatus of forming an image by ejecting a liquid functional material on a recording medium by an inkjet head.
2. Background Art
The inkjet apparatus of ejecting a liquid functional material (hereinafter referred to as an “ink”) on a recording medium by using an inkjet head and thereby performing the image formation is characterized, for example, in that the system is environment-friendly, high-speed recording on various recording mediums can be realized, and a high-resolution image with less blurring can be obtained. Above all, development of an apparatus using an ultraviolet-curable ink is proceeding in view of easy handling of the light source, compactness and the like.
However, when an air bubble or dissolved gas is present in the inkjet ink, the ink cannot be satisfactorily compressed at the ejection and this gives rise to a problem of deterioration in the ejection property, incapability of generating stable ink drops, occurrence of dot missing or printing failure, or reduction in the reliability of the inkjet recording method.
For this reason, in the invention of JP-A-5-17712 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) or JP-A-2004-196936, degassing of the inkjet ink is generally performed to remove the air bubble or dissolved gas.
FIG. 5 shows an inkjet recording apparatus previously developed by the present applicant based on the invention of Patent Document 1. In the Figure, 50 is an image recording part, 52 is a head unit, 58 is an inkjet head, 70 is an ink supply part, 72A and 72B are ink tanks, 74 is a pump equipped with a filter, 75 is a pipe as an air-feeding mechanism, 76 is a degassing (deoxygenating) device, 78 is a vacuum pump, 80 is an ink supply path, 80a is a guiding branch, 84 is an air-exposure valve, and L′ is an aqueous ink (or a solvent ink).
This inkjet recording apparatus works as follows.
At the image formation, the air-exposure valve 84 is closed and the degassing mechanism 82 is simultaneously actuated to degas the aqueous ink L′ in the ink supply path 80 and supply the ink to the inkjet head 58. The degassed aqueous ink L′ is ejected from the inkjet head 58 based on the control of a head driver 54 and landed on a recording medium S (FIG. 1), and an ultraviolet ray is irradiated thereon from an ultraviolet irradiating part 56 (FIG. 1), whereby an image is formed.
The aqueous ink L′ landed on the recording medium S is degassed and therefore, there is obtained an effect that the generation of a bubble in the inkjet head 58 is reduced and the ejection failure ascribable to the bubble is improved.
However, the ink used here is an aqueous ink or solvent ink and such an ink contains a component having a boiling point of about 100° C. In the case where the boiling point of the component is 100° C., boiling occurs when the pressure is reduced to about 0.1 atm at room temperature, and the occurrence of boiling is accompanied with alteration of the ink component. Therefore, degassing to a high vacuum of 0.1 atm or less (for example, 0.06 atm or less) is substantially impossible.
On the other hand, the gas component in the ink grows by rectified diffusion and particularly when the ejection is continuously performed for a long period of time, the conventional degassing to about 0.1 atm cannot stop growth of the bubble and allows for occurrence of an ejection failure and thus, this is insufficient. There has been no means effective for stably performing the inkjet ejection for a long period of time.